The purpose of this application for a supplement in response to PAR-03-063 ("NHLBI Competitive Supplements for Human Embryonic Stem Cell Research") is to enable my lab, which has little prior experience working with human embryonic stem cell (HESC) lines, to enhance our existing aims of my ongoing NHLBI grant (R2.1 HL72229: "Fas ligand* blood cells to reduce transplant rejection"), which is now being pursued using primary mouse and human lympho-hematopoietic stem-progenitor cells (HSCs). The overall goal of my ongoing grant is to investigate the ability of lentivirally (LV) transduced HSCs or dendritic cells (DCs) expressing high levels of Fas ligand (FasL) to reduce the response to alloantigen (Ag) and increase engraftment of allogeneic (allo) HSCs. In Aims 1-2 of this proposal, we proposed to investigate this in cellular immunology and transplant experiments, as well as to elucidate the apoptotic pathways in alloimmune cells. Since transduced FasL [unreadable] HSCs might potentially be toxic in vivo, we proposed to investigate technologies to limit potential FasL toxicity. A novel FasL [unreadable] cell therapy approach to reduce graft rejection might eventually be used in clinical allo blood and marrow transplantation (BMT). Furthermore, achievement of stable lympho-hematopoietic chimerism would be predicted to generate tolerance for transplanted allo organs, which is explored in Aim 3. HESCs can be manipulated to generate blood cells, and hematopoietic transplants of HESC-derived HSCs have been proposed with proof-of-principle illustrated in mice. Since immunologic rejection is a major barrier to use of allo HESCs (including HESC-derived HSCs) clinically, a next logical step for us is to investigate whether FasL transduced HESC-derived HSCs/DCs might reduce the alloimmune response. This use of HESC lines is clearly within the scope and a logical extension of the goals and objectives of our ongoing grant. Indeed, we did not propose this in the initial application only because we had absolutely no hands-on experience with HESCs at that time, and HESC lines were then extremely difficult to obtain. In this supplement application, the experiments involving HESCs are confined to Specific Aim 2.2 (Hypothesis 2.2: FasL [unreadable] HSCs and their progeny will delete attacking alloreactive effector T cells -- extended in this supplement to investigate HESCs-derived "HSCs"). [unreadable] [unreadable]